Plates of a hull steel were weld via CO₂ gas-shielded arc welding with marine high strength flux-cored wire as filler and by three different heat inputs i.e. 8 kJ/cm,14 kJ/cm and 20 kJ/cm respectively, while the effect of heat input on the microstructure and low-temperature flexibility of the weld seams was investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and materials-electron backscatter diffraction. Results show that the microstructure of the weld metal consists of mainly acicular ferrite, ferrite side-plate and a small amount of residual austenite for three different heat inputs. As the heat input increases the ferrite changes from acicular to lath, in which the acicular ferrite content decreases, side-plate ferrite increases and the residual austenite between them also changes from film-like to block. In addition, with the increasing heat input, inclusions with diameter below 1 μm in the deposited metal decrease, while the total amount of inclusions increases, and the large angle grain boundaries between the strips decrease. Consequently, the low-temperature flexibility of the weld seam decreases, and the fracture surface also transformed from dimple- and quasi cleavage-like to cleavage-like.